In the prior art there is known a variety of methods for the production of cubic boron nitride (.beta.--BN), which comprise subjecting hexagonal boron nitride (.alpha.--BN) to an elevated temperature and pressure in the presence of at least one catalyst selected from the class consisting of alkali metals, alkaline earth metals, and nitrides of the foregoing metals (see, for example, U.S. Pat. No. 2,947,617 patented on Jan. 6, 1958).
The cubic boron nitride produced by these methods does not exhibit a sufficiently high level of strength, especially in moist media, due to the formation of considerable amounts of by-products located in the grains of cubic boron nitride, in particular, the nitrides of a catalytic metal, arising in the process of synthesis, are especially susceptible to the effect of moisture.
Furthermore, the process of manufacture of abrasive articles from the cubic boron nitride produced by the above prior art methods is accompanied by partial destruction of cubic boron nitride grains and of the abrasive product itself on account of the tendency of nitrides of a catalytic metal to oxidation.
According to the currently adopted terminology the substances which are conductive to the formation of cubic boron nitride are called catalysts. However, in the final product, i.e. cubic boron nitride, these substances are not encountered in pure form, therefore, the substances favoring the formation of cubic boron nitride will be hereinafter referred to as "initiators".
Known in the prior art is a method of producing cubic boron nitride at elevated temperatures and pressures from charges including hexagonal boron nitride and conversion initiators, the latter being comprised of borides of alkali and/or alkaline earth metals taken in combination or separately (see, for example, U.K. Pat. No. 1,335,909 patented on Apr. 19, 1971).
When practicing the above-described method for the production of cubic boron nitride, the yield of a target product in the form of a powder with a grain size of more than 100 mcm, micrometer, (this is the type of powders being most extensively utilized for the manufacture of abrasive tools) amounts all in all to about 20%.
Also known in the prior art is a method of producing cubic boron nitride, which comprises mixing hexagonal boron nitride with 0.5 to 7 weight percent of a member selected from the group consisting of phosphorus, a phosphorus compound and mixtures of at least two such materials, subjecting the resulting mixture to elevated temperatures and pressures, under which the cubic crystalline boron nitride structure is stable, and sequentially reducing the temperature and lowering the pressure of said mixture, lithium hydride and lithium nitride being used as a conversion initiator (see, for example, U.S. Pat. No. 3,881,890 patented on Apr. 20, 1973).
The use of the foregoing materials decomposable in air as a conversion initiator involves significant complexities in the preparation of an initial charge for cubic boron nitride synthesis and does not afford the possibility of mechanizing the procedure of charge preparation in the conditions of commercial manufacture. Moreover, in order to obtain an adequately high yield of commercially desirable powders with a grain size of more than 100 mcm there is required a fairly protracted synthesis time interval (up to 20 min).
The completeness of conversion of hexagonal boron nitride to cubic boron nitride as well as the number of crystals with a grain size of more than 100 mcm can be augmented, provided the crystallization medium is transformed in conformity with the present invention.